1. Statement of the Technical Field
The invention concerns dielectric substrates for RF circuits, and more particularly dielectric substrates with effective permittivity values that can be independently controlled in predetermined portions of the substrate.
2. Description of the Related Art
The use of mobile telephones, such as cellular telephones, has become pervasive throughout much of the world. While being operated, most modern cellular telephones are held very close to the human body, for example next to a user's ear or on the user's belt. Cellular telephones typically interface with communications networks by receiving and transmitting low power RF signals through a dipole antenna. However, such signals are often disrupted by the proximity of the antenna to the human body. In particular, current state of the art antennas produce near electric fields that couple to the polar water molecules in human tissue, thereby reducing signal strength. For example, human tissue can attenuate a 960 MHz RF signal transmitted by a conventional dipole antenna at a rate of 6 dB per inch.
Further, many experts believe that the interaction of the RF signals with a person's tissue can have dangerous health risks. Some contend that the RF signals can interfere with the body's natural electrical systems. This reaction can vary depending on the individual, but there is speculation that the RF signals can harm a person's immune system and spur cancer development. It also has been alleged that RF signals from cellular telephones can interfere with brain activity, accounting for the symptoms of memory loss, changes in blood pressure, anxiety and lack of concentration. Accordingly, there exists a need for an antenna that can be used in mobile communications systems to improve RF signal propagation and reduce the interaction between RF signals and the human body. Moreover, there exists a need for an antenna that will operate with low VSWR, stable tuned frequency, and high efficiency when the antenna operates near water and moist soils.